Effects of Nanoporous Structure of Anodic Films on Adhesive Strength betweenAluminum Alloys and Polyamide Resin+1

Kota Sato1,+2, Hidetaka Asoh2,+3 and Hitomi Yamamoto3

1Applied Chemistry and Chemical Engineering, Graduate School of Engineering, Kogakuin University, Tokyo 192-0015, Japan2Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University, Tokyo 192-0015, Japan3Mitsubishi Aluminum CO., LTD., Susono 410-1127, Japan

In this study, the effects of the nanoporous structure of anodic films on adhesive strength between aluminum alloys and polyamide resinwere systematically investigated. Alumina films with different dimensions (such as pore density, diameter, and depth) were formed on A6063aluminum alloys by various anodizing conditions to compare the anchoring effect. The adhesive strength at the interface between the adherend(anodized aluminum) and adhesive (thermoplastic elastomer resin) was evaluated by a method for determining the tensile lap-shear strength ofrigid-to-rigid bonded assemblies. The higher pore density and larger pores in anodic films were important factors for improving the adhesivestrength and increasing the adhesion interface area and the amount of adhesive impregnated into the pores. After anodizing in phosphoric acid at60V and subsequent pore widening, the adhesive strength of aluminum was 17.4MPa, which was ³3.5 times higher than that of an aluminumsubstrate without surface treatment. [doi:10.2320/matertrans.MT-L2021006]

(Received June 28, 2021; Accepted September 16, 2021; Published November 5, 2021)

Keywords: aluminum, anodizing, porous anodic film, adhesive strength

1. Introduction

As a solution to recent energy problems, weight reductionsin structures are desired; thus, lightweight and highlyworkable aluminum alloys are being used to manufacturevarious products, such as automobiles and aircrafts.1­4) Whilefabricating products from processed parts, combinations ofmaterials are diverse; thus, not only conventional mechanicalfastening technology (such as bolts and caulking) is used, butadhesion is also used as a joining technology that does notuse surplus parts, such as screws and rivets.5) When adheringtwo materials using an adhesive, the adhesion strength varieson a surface of smooth aluminum alloy; thus, their surfacesare etched or altered by polishing or chemical processingto stabilize or to improve the adhesive strength through theanchoring effect and chemical bond. This method has beenused for a long time.6­10)

An anodizing process with a low environmental burdengenerates alumina film with a porous structure on analuminum surface.11­13) The generated film increases thealuminum alloy’s corrosion resistance and improves theadhesive strength as the resin impregnates the pores.14­19)

Muraoka et al. investigated the tensile shear strength ofA5052 aluminum alloy treated with H3PO4 electrolytesolution and polyethylene resin, and measured the specificsurface area of the anodic porous alumina film (i.e., theadhesion interface) with the gas adsorption method. Theresult showed that a larger specific surface area of theprepared film, i.e., deeper pores and higher pore density,meant higher adhesive strength.15) Nagato et al. fluorinatedthe surface of A5052 aluminum alloy, which was treated withH3PO4 electrolyte solution, through plasma processing andassessed its adhesive strength with epoxy resin using the

T-peel test by dividing the effects into anchoring effect andchemical bond (JIS K 6854-2). Samples with high adhesivestrength had large pore diameters. Scanning electronmicroscopy (SEM) study showed a large remnant of resinon the peeled surface. They reported that the ratio of thecohesive failure of resin exceeded that of interfacial failure.Also, a simulation using the finite element method showedthat the cohesive failure of resin increased as the porediameter increased; thus, it was argued that larger and deeperpores caused the anchoring effect and improved the adhesivestrength.16) Furthermore, Zhang et al. adjusted the H2SO4

concentration of the H2SO4­H3PO4 mixture electrolytesolution, used 2024 aluminum alloy as the test material,and assessed the adhesive strength of the samples withvarious porosity values (ratios of pore volume to the totalvolume of the oxide films) with an adhesive using the pull-offadhesion test (ISO 4624). However, it was claimed thatincreased porosity reduced the film’s hardness, but improvedthe adhesive strength. Besides, the film’s thickness wasuniform (approximately 42 µm), and the diameter of thepores examined was <100 nm. Thus, this study cannot beconsidered universal.17) Additionally, methods such as asurface preparation method, which uses AC electrolysis ofaluminum alloy in an alkaline electrolyte solution,18) andtwo-step anodizing process, which uses different electrolytesolutions,19) are being considered.

Generally, films with large pore diameters prepared withH3PO4 electrolyte solution are considered suitable surfacesfor coating and adhesion,20) but many unknowns existregarding the impact of the film’s pore diameter, density,and depth on the adhesive strength. Since the anodizingbehavior and film structure vary based on the type of an alloyused, a systematic examination of the relationship betweenthe pore structures of films prepared on various aluminumalloys and its adhesive strength with resin is significant ingaining further understanding of the essentials of adhesion.Herein, we used A6063 aluminum alloy samples withsuperior extrusion processability and assessed the adhesive

+1This Paper was Originally Published in Japanese in J. JILM 71 (2021)234­240. The abstract and the caption (Table 2 and Figs. 3­5) of thispaper are slightly modified.

+2Graduate Student, Kogakuin University+3Corresponding author, E-mail: asoh@cc.kogakuin.ac.jp

Materials Transactions, Vol. 62, No. 12 (2021) pp. 1724 to 1731©2021 The Japan Institute of Light Metals

strength of the samples prepared with the film’s pore size,density (interpore distance), and depth (thickness) regulatedby anodizing conditions through a tensile shear strength testto systematically show the impact of the pore structure ofanodic film on adhesive strength.

2. Experimental

2.1 Preparation of specimensTable 1 shows the chemical composition of the A6063-T3

aluminum alloy used as test material. Strips of dimension100mm © 25mm © 1.5mm were cut out from the testmaterial and were degreased as a preprocess for 5min byultrasonic cleaning in acetone. Subsequently, to fix thereaction surface at about 2,000mm2, a polytetrafluoro-ethylene tape was attached to a position 38mm from thebottom edge. In the various electrolyte solutions (H2SO4 andH3PO4) shown in Table 2, constant voltage anodizing wasperformed at a temperature of 25 « 1°C (20 « 1°C only for180V) and applied voltage of 10­180V using a DC powersupply (Takasago Ltd. GP0500-1R). The electrolysis currentwas measured at intervals of 0.1 s using a digital multimeter(Keithley 2700). Anodization was performed through the

constant voltage electrolysis to regulate the pore diameterand interpore distance. Here the quantity of electricitynecessary to prepare a 2 µm thick film was determined froma preliminary study result, and the electrolysis duration wasadjusted for each type of electrolyte solution with thenecessary energy as the guide. Several samples wereimmersed in 5 or 10mass% H3PO4 at 30°C after preparingthe oxide film to enlarge pore diameters. Also, some samplesunderwent constant voltage electrolysis again at a differentvoltage as two-step anodizing process.

2.2 Observation of film structuresThe film structure was observed using field emission

scanning electron microscopy (FE-SEM, JEOL JSM-6701F),and energy dispersive spectroscopy (EDS, AMETEK EDAX-EDS system) was used for elemental analysis. Cross-sectionsamples for EDS analysis were prepared using a focused ionbeam system (FIB, Hitachi NX2000).

On the surface SEM images of the oxide film, ten locationswere selected randomly to measure the pore diameters, ofwhich the mean value was used as the film’s pore diameter.Additionally, the film thickness was measured using thecross-sectional SEM image of the oxide film. Furthermore,the distance between the centers of the adjacent cells’ basein five locations was measured and used as the mean as theinterpore distance.

2.3 Adhesive strength testTo assess adhesive strength between the aluminum alloy

and resin, tests were conducted based on JIS K 6850

Table 1 Chemical composition of A6063 aluminum alloy (mass%).

Table 2 Anodizing conditions and dimensions of anodic films formed on A6063 aluminum alloy.

Effects of Nanoporous Structure of Anodic Films on Adhesive Strength between Aluminum Alloys and Polyamide Resin 1725

(Adhesives­Determination of tensile lap-shear strength ofrigid-to-rigid bonded assemblies) using a precision universal/tensile tester (Shimadzu Corporation AG-X 100 kN). Thismethod is widely used to assess the adhesive strength betweenresins, adhesives, and paints. Additionally, tensile forceparallel to the major axis and the adhesion part of the testspecimen was applied to the adherend, and a shear-directionburden was applied to the simple overlap part between rigidadherends to measure the shear adhesive strength.

A polyamide-based thermoplastic adhesive film (NihonMatai Elfan μNT) was inserted between samples thatunderwent constant voltage electrolysis under the sameconditions such that the adhesive area in the overlap areawas 3.13 cm2. Using hot press at 160°C, the sample wascompression bonded for 5min at 62.8 kPa. Subsequently, itwas slowly cooled to 100°C or below. Then, the load wasremoved, and the sample was cooled to room temperature.Then, the adhesive strength test was performed four times.Here, eight samples prepared under the same conditions(adherends) were divided into four groups of two samplesfor measurements. Out of four measurements, the mean valuewas taken from three results while excluding one outlier,which was used as the shear adhesive strength. Forcomparison, an A6063 aluminum alloy sample that was onlydegreased was used.

2.4 Observation of samples after lap-shear testPeeled surfaces of the samples used for the adhesive

strength test were divided into the resin/film interface, thefilm surface, and the resin side for SEM observation. In thismanner, fracture morphologies were observed.

3. Results and Discussion

3.1 Preparation of anodic film by anodization atconstant voltage

Figure 1 shows the current density-time curves of theconstant voltage electrolysis performed on the samples. Forall samples, the current reached the upper limit of the outputpower supply (500Am¹2) during the barrier layer formationat the initial electrolysis stage. Subsequently, it quicklydecreased and increased again, which represents the typicalelectrolysis behavior that indicates a porous layer formation.Samples that underwent the constant voltage electrolysis inH2SO4 electrolyte solution at 10V or H3PO4 electrolytesolution at 60V (samples a, b, e, and f ) showed a constantcurrent of 40Am¹2. The quantity of electricity used duringanodization to prepare a 2 µm thick film was 40­52 kCm¹2

under all conditions with good reproducibility during samplepreparation. However, samples c, g, d, and h that underwentconstant voltage electrolysis in H3PO4 electrolyte solution at120 or 180V, did not reach a constant current during theelectrolysis, where the current continuously increased till theend of the electrolysis.

3.2 SEM observation of anodic filmsFigure 2 shows the prepared film’s surface and cross-

sectional SEM images. Table 2 shows the experimentalconditions along with pore diameters, interpore distances,and film thickness.

3.2.1 Dependence of pore density (interpore distance)Sample a, which was anodized in the H2SO4 electrolyte

solution, had a pore diameter of approximately 12 nm.However, irregular open-pore-shaped dimples of approx-imately 90 nm were also found, which possibly stem from thevarying solubility of the oxide film and localized currentconcentration due to the distribution of alloy components.Additionally, the cross-sectional SEM images showed a filmwith a uniform thickness of approximately 2.1 µm, whichconfirms a typical porous structure with pores perpendicularto the film’s surface with an interpore distance ofapproximately 27 nm (Fig. 2(a)). Samples b, c, and d, whichwere prepared in the H3PO4 electrolyte solution, wereobserved at the same magnification as in Fig. 2(a). Thesesamples presented relatively large pores with diameters ofapproximately 99, 96, and 96 nm. Interpore distances weremostly proportional to generating voltages of 60, 120, and180V during sample preparation: approximately 150, 300,and 440 nm, respectively. The film’s cross-sectional SEMimages confirmed truncations and branching of pores (arrowsin Figs. 2(b)­(d)). As clearly seen in Figs. 2(c) and 2(d),pores (initial pores) on the film’s surface were smaller thanuniform pores inside the film. This trend shows that betweenthe surface and internal areas of the film, the number of poresper unit area, i.e., pore density, was different, where thedensity decreased inside the film. The porosity of a porousalumina film prepared under the self-ordering condition,where pores are arranged regularly over a long distance,was approximately 10%,21,22) indicating that the ratio of thepore diameter to interpore distance is approximately 1/3.However, the pore diameter/interpore distance ratio ofsample b prepared in the H3PO4 electrolyte solution at 60Vwas 0.63, where the porosity was estimated to beapproximately 40%. However, samples c and d prepared inthe H3PO4 electrolyte solution at 120 or 180V, respectively,had porosity values of 10 and 4.8%, respectively, indicatingthat films prepared with a higher voltage had lower porosityvalues and thicker cell walls.

Fig. 1 Current density­time curves for anodization of A6063 in (a)1mol dm¹3 H2SO4 at 10V, (b), (e), (f ) 1mol dm¹3 H3PO4 at 60V, (c),(g) 0.5mol dm¹3 H3PO4 at 120V, and (d), (h) 0.1mol dm¹3 H3PO4 at180V.

K. Sato, H. Asoh and H. Yamamoto1726

3.2.2 Dependence of pore diameter and porositySamples e and f are sample b with pore widening

treatment performed for 30 and 60min, respectively. Thepore diameters on the films’ surfaces of these two sampleswere widened to 142 and 166 nm (Figs. 2(e) and (f )).Samples g and h are samples c and d with pore wideningtreatment. Their pore diameters were widened to approx-

imately 206 and 331 nm (Figs. 2(g) and (h)). Compared tothe original samples, none of the samples that underwent thepore widening treatment showed a change in the interporedistance (pore density) with only a slight difference in thefilm thickness (pore depth). However, the cell wall becamethinner, which increased the porosity. Samples f and hpresented horizontal openings (side holes) that connected

Fig. 2 Surface and cross-sectional SEM images of anodic oxide films with different porous structures. The insets in (a) indicate highmagnification images of porous structures marked by the square in each image. Anodizing was conducted under the conditions shown inTable 2. The scale bar applies to images in the figure, both in surface and cross-sectional images.

Effects of Nanoporous Structure of Anodic Films on Adhesive Strength between Aluminum Alloys and Polyamide Resin 1727

adjacent pores each other where a part of the porous layer cellwall had dissolved (arrow in Figure). Furthermore, as a partof the barrier layer dissolved, through-holes formed, leadingto localized areas where the substrate had dissolved at thefilm/substrate interface, creating cavities at the bottom of thefilm.3.2.3 Dependence of film thickness

Sample i was prepared by reducing the electrolysisduration by half (14min) under the same conditions as withsample b. Meanwhile, sample j was prepared by doublingthe electrolysis duration (56min). Their film thicknesses wereapproximately 1.1 and 3.9 µm, respectively. Since the filmthickness of sample b (28min) was approximately 2.3 µm,it was confirmed that the film thickness increased almostproportional to the electrolysis duration (Figs. 2(b), (i), and( j)). Pore diameters of samples i, b, and j were approximately78, 99, and 138 nm. Regardless of the generating voltagebeing 60V for all, a longer electrolysis duration increased theamount of dissolution of the film by the acidic electrolytesolution, which increased the pore diameter on the outermostsurface.3.2.4 Dependence of porous structure

To further examine the impact of the anchoring effect onadhesive strength, sample k was prepared with a 2 µm thicktwo-layer structure with different porosities, in which theupper layer (1 µm) was prepared based on the preparationconditions of sample b (60V) and the lower layer (1 µm) onthe preparation conditions of sample c (120V). The porediameter at the top surface of sample k was approximately80 nm, and there was an unevenness on the film’s surfacefrom chemical dissolution during electrolysis (Fig. 2(k)).Cross-sectional SEM images showed that in the upper layer,cell thickness and pore branching like that of sample b wereobserved. Some pores stopped their growth from the top tothe bottom layer, whereas others grew continuously, forming

the lower layer. In the lower layer, a cell wall exists with athickness like sample c. The number of pores in the upperlayer was about four times that in the lower layer. It can besaid that this sample has a porous structure where the porediameter was larger inside the film.

3.3 Evaluation of degree of resin impregnation onanodic film by EDS

Chemical composition of the film after the resin adhesionwas analyzed using EDS. Figure 3 shows the typical result ofthe elemental mapping (Al, O, and C). In sample a with apore diameter of approximately 12 nm, the resin componentC (carbon) was not detected in the film (Fig. 3(a)), which wasattributed to the small pore sizes on the film’s surface andresin not impregnating the film’s interior. In contrast, insamples b, c, and g with pore diameters of 96 nm or more, Cwas detected up to the bases of pores (Figs. 3(b)­(d)). Thus,it was confirmed that in a 2 µm thick film, if the pore sizewas 100 nm or more, the resin can impregnate to the base ofthe pore. In samples b and c with similar pore diameters, Cwas detected at a higher level in sample b with higher poredensity than in sample c. Thus, it was assumed that higherpore density allows for more infiltration of resin in the film,increasing the resin/film interface area.

3.4 Adhesive strength test3.4.1 Effect of pore density and pore diameter (porosity)

Figure 4(a) shows the result of the adhesive strength testfor all samples measured in this study. Figure 4(b) shows thatwhen the adhesive strength of each sample was compared tothe adhesive strength of the non-anodized substrate (shownas “Sub.” in the graphs) to resin (5.0MPa), the adhesivestrength values of the anodized samples, a, b, c, and d, were9.1, 10.1, 8.3, and 5.3MPa, respectively, any of which ishigher than that of the untreated substrate. If we assume that

Fig. 3 Cross-sectional EDS maps of selected samples (Samples a, b, c, and g). Anodized aluminum specimens were prepared under thefollowing conditions: (a) 1mol dm¹3 H2SO4 at 10V, (b) 1mol dm¹3 H3PO4 at 60V, and (c), (d) 0.5mol dm¹3 H3PO4 at 120V.

K. Sato, H. Asoh and H. Yamamoto1728

the adhesive strength of the substrate (5.0MPa) is attributedto the chemical bond between the aluminum surface and resinalong with the unevenness of the substrate, the increase overthe reference value can be considered due to the anchoringeffect of the porous structure. Thus, adhesive strength wasimproved by the resin impregnating the dimples with 90 nmdiameter in sample a, and the resin impregnating the poreswith a diameter of 96 nm or more in samples b, c, and d.Furthermore, since the pore density increased in the orderof samples d < c < b while their pore diameters remainedalmost constant, a trend existed where the higher pore densitymeant a higher area of the adhesion interface (where the totalarea equals the film’s surface area plus the internal area ofpores) and higher adhesive strength, similar to the result byMuraoka et al.15)

Figure 4(c) shows that the adhesive strength of sample e(pore diameter of 142 nm), which was prepared by applyingthe pore widening treatment on sample b for 30min, was10.1MPa, which was like that of sample b. However, theadhesive strength of sample f, which underwent 60min ofthe pore widening treatment (pore diameter of 166 nm), wasthe highest of all samples examined in this study (17.4MPa).This value was equivalent to 3.5 times the adhesive strengthof the substrate (reference value) without the porous film.Similarly, the adhesive strength of sample g (pore diameterof 206 nm), which was prepared by applying the porewidening treatment on sample c for 60min, was 15.5MPa,which was about three times the reference value. Theadhesive strength of the sample h (pore diameter of331 nm), which was prepared by applying the pore wideningtreatment on sample d for 90min, was 7.7MPa, which was

higher than that of sample d. Therefore, as long as the poredensity is constant, larger pores in anodic films led to higheradhesive strength, which was consistent with the results ofNagato et al.16)

Therefore, if the pore diameter remains constant, higherpore density provides higher adhesive strength, and if thepore density is constant, higher porosity provides higheradhesive strength, which agrees with the result of Zhanget al.17) In other words, higher porosity allows more resinto impregnate the pores, which increases the mechanical orchemical bond at the interface. In addition to the findings ofthe previous studies,15­17) this present result clearly showedthat the adhesive strength is most strongly impacted by thepore structure of the adherend (i.e., porous alumina film),regardless of the type of aluminum alloy and adhesive.3.4.2 Effect of pore depth

Figure 4(d) shows that the adhesive strength of samples i(film thickness of 1.1 µm), b (2.3 µm), and j (3.9 µm) were12.1, 10.1, and 9.7MPa, respectively, which all exceeded thatof the substrate (reference value = 5.0MPa). However, therewas no notable difference in the adhesive strength basedon the difference in the pore depth between 1 and 4µm(Fig. 4(d), samples i, b, and j). Thus, in films with porediameters of less than 140 nm, even if the resin isimpregnated at the base of the pore, the porous structure ofthe top 1 µm of the sample surface was the determining factorof the adhesive strength.3.4.3 Anchor effect of two-layer structure

Figure 4(d) shows that the adhesive strength of sample kwith a two-layer structure and different pore density valueswas 12.9MPa, which is like the adhesive strength of sample i

Fig. 4 Adhesion strength of anodic oxide films with different porous structures: (a) overall comparison and effects of (b) pore distance,(c) porosity, and (d) film thickness. Anodized aluminum specimens were prepared under the conditions shown in Table 2. Each value isthe average of three measurements.

Effects of Nanoporous Structure of Anodic Films on Adhesive Strength between Aluminum Alloys and Polyamide Resin 1729

(12.1MPa). This finding supported the above-describedresult where the film structure of the top 1 µm affected theadhesion. Besides, considering the film thickness of 2 µm,compared to the adhesive strength of samples with less porebranching (samples b (10.1MPa) and c (8.3MPa)), sample kwith a two-layer structure had higher adhesive strength. Inaddition to the anchoring effect where the resin infiltratedsample k with complex pore morphology, the unevenness ofthe film’s surface improved the adhesive strength.

3.5 Observation of peeled surface of samples after lap-shear test

Figure 5 shows the post-adhesive strength test appearanceand surface SEM image of sample b and sample g, whichshowed relatively high adhesive strength. The appearanceconfirmed that both samples had residual resin on the entirepeeled surface (adhesive side). Thus, it was assumed that thecohesive failure of the resin was the main failure factor. TheSEM observation also confirmed the cohesive failure of theresin, the same result by the outer appearance observation.The failure of the film, i.e., failure on the side of theadherend, was not observed. However, the residual resin andthe porous structure of the film’s surface were observed onthe same plane, confirming that some resin extends in thetensile direction from the pores in the film (arrows inFigs. 5(a)-3 and (b)-3). This trend indicates that an interfacialfailure exists at the resin/film interface while the cohesivefailure dominates.

There were markings of failure on the resin’s surface wherethe resin that impregnated the pores was stretched to failure(arrows in Figs. 5(a)-4 and (b)-4). The cohesive failure ofthe resin that impregnated pores also affected the adhesivestrength. Therefore, improving the adhesive strength requiresincreasing the adhesive area and the anchoring effect, andalso increasing the film’s porosity to increase the resinimpregnation to improve the interface mechanical orchemical bond. Since there were little variations in theobservation results of the peeled surface and the adhesivestrength test results, the adhesion was considered good, andthe failure was mainly cohesive within the resin layer.

4. Conclusions

With A6063 aluminum alloy test material, we observedthe pore structure of the prepared samples with SEM andassessed the adhesive strength with the tensile strength testto clarify the impact of the pore structure of the anodic filmon the adhesive strength of resin. The following conclusionswere reached:(1) Eleven samples were prepared with different film pore

diameter, density (interpore distance), and depth (thick-ness) values with H2SO4 or H3PO4 electrolyte solutions.In adhesion with a polyamide resin, it was confirmedby EDS analysis that by a formation voltage of 60Vor more and a pore diameter of 100 nm or more, resincould impregnate to the base of the pores.

(2) The adhesive strength values of all anodized sampleswere higher than the adhesive strength of the non-anodized substrate (5.0MPa) because of the anchoringeffect of the porous structure of the adherend surface.

(3) Among the samples with similar pore sizes preparedin the H3PO4 electrolyte solution under 60­180V,samples prepared under 60V with higher pore densityhad higher adhesive strength (10.1MPa). An increasein the adhesive area of the adhesion interface (the totalarea that combines the film’s surface area and areainside of pores) effectively improved the adhesivestrength.

(4) All samples that underwent the pore widening treatmenthad adhesive strength values that were 1.7­1.9 timeshigher than that of the untreated samples. The samplewith the highest adhesive strength (17.4MPa) in thisstudy had a film thickness of 1.9 µm with a porousstructure and a pore diameter of 166 nm. Anodizationimproved the adhesive strength by 3.5 times comparedto that of the untreated substrate.

(5) Samples with film thicknesses of 1­4µm that wereprepared in the H3PO4 electrolyte solution showed nodifference in the adhesive strength based on the poredepth. For films with a pore diameter of less than140 nm, the upper-most porous structure within 1 µm of

Fig. 5 SEM images of samples (a) b and (b) g after lap-shear test.

K. Sato, H. Asoh and H. Yamamoto1730

the surface was the determining factor of the adhesivestrength.

(6) The adhesive strength of the sample with a two-layerstructure with varying pore density (12.9MPa) washigher than that of the sample with less pore branching(10.1MPa). The anchoring effect, expressed as the resinimpregnated the film with a complex pore morphology,contributed to improving the adhesive strength.

(7) Observation of the peeled surface after the adhesivestrength test and high reproducibility of the adhesionstrength test results indicated that the adhesion betweenthe polyamide resin and adherend was good, and failurewas mainly cohesive within the resin layer. Thus, toimprove the adhesive strength, it is important toincrease the adhesive area and anchoring effect by poreformation, and also to increase the film’s porosity toimprove resin impregnation, which would improve theinterface mechanical and chemical bonds.

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